Powerplants for vehicles, such as the propulsion system for an aircraft, use a propulsion system assembly which includes an engine disposed in a pod or nacelle. The engine has a flow path for working medium gases which extends through the engine and from the exit of the engine. As these hot, high velocity gases exit the engine and enter the atmosphere, the gases may cause unacceptable levels of noise in the exhaust.
The amount of noise generated by the working medium gases as they pass through the atmosphere is proportional to their velocity. Accordingly, many designs exist for mixing cool, relatively low velocity gases from the exterior of the engine with the hot, high velocity gases to decrease the level of noise. One example of such a design is shown in Department of Transportation (FAA) Publication entitled "727 Noise Retrofit Feasibility, Volume 2" by Anderson et al of the Boeing Company, dated November 1972. This final report shows in FIG. 2-8 the use of an ejector shroud in combination with a lobe assembly to mix gases from the exterior with gases from the interior.
U.S. Pat. No. 3,696,617 issued to Ellis is another example of a sound suppression device employing the mixing of cool exterior gases with hot working medium gases from the interior. In this embodiment, the ejector is spaced axially from the nacelle leaving an opening therebetween. A pylon 12 for supporting the engine nacelle and shroud extends longitudinally above the engine nozzle and shroud and partially blocks the flow of cool exterior gases into the ejector.
The above art notwithstanding, scientists and engineers working under the direction of Applicants' Assignee are still seeking to improve propulsion system assemblies to decrease levels of noise associated with the exhaust stream.